fix(proxy): add missing timezone import

The proxy_server.py uses timezone.utc in several places but timezone
was not imported from datetime module, causing mypy type errors.

Lines affected: 8227, 8229, 8237, 8239, 8276, 8283

litellm/proxy/proxy_server.py

@@ -11,7 +11,7 @@ import sys
 import time
 import traceback
 import warnings
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 import enum
 from typing import (
     TYPE_CHECKING,